Fracture plugs, bridge plugs, and the like are used in a tubular to block off flow. A fracture plug is used to seal fluid pressure from above, whereas a bridge plug is used to seal from above and below. Typically, the plugs have mandrels and other components composed of a millable material, such as a composite material. Seals on the mandrels can be compressed to seal inside the tubular, and slips are typically used on the plug to engage the plug inside the casing. Once the plug has been used for its purpose, it is typically milled out in a milling operation.
In many cases, the plugs have metal slips. These metal pieces cause issues during milling, and the metallic residue may not readily flowback to the surface. For this reasons, composite plug providers have tried to reduce the amount of metal in the composite plugs.
Slips used for a composite plug can be composed of metal, such as cast iron, or they may be composed of composite materials with inserts or buttons disposed in the slip to grip the inner wall of a casing or tubular. Examples of downhole tools with slips and inserts are disclosed in U.S. Pat. Nos. 6,976,534 and 8,047,279.
As shown in FIG. 1, a typical composite plug P has a mandrel 10 with cones 14 and backup rings 16 arranged on both sides of a packing element 18. Outside the inclined cones 14, the plug P has slips 12a-b. The slips 12a-b can be a conventional wicker slip (as with slip 12b) composed of cast iron or can be a composite material slip (as with 12a) having inserts or buttons 13. The composite plug P is preferably composed mostly of non-metallic components according to procedures and details as disclosed, for example, in U.S. Pat. No. 7,124,831, which is incorporated herein by reference in its entirety. This makes the plug P easy to mill out after use.
When deployed downhole, the plug P is activated by a wireline setting tool (not shown), which uses conventional techniques of pulling against the mandrel 10 while simultaneously pushing an upper component 15, which pushes against the upper slip 12a and forces a head 11 against the lower slip 12b. The force used to set the plug P may be as high as 30,000 lbf. and could even be as high as 85,000 lbf. These values are only meant to be examples and could vary for the size of the plug.
As a result, the slips 12a-b ride up the cones 14, the cones 14 move along the mandrel 10 toward one another (because the components are being pushed downward on the mandrel 10 against the fixed head 11), and the packing element 18 compresses and extends outward to engage a surrounding casing wall. The backup elements 16 control the extrusion of the packing element 18. The slips 12a-b are pushed outward in the process to engage the wall of the casing, which both maintains the plug P in place in the casing and keeps the packing element 18 contained.
Once set, the plug P isolates upper and lower portions of the casing so that fracture and other operations can be completed uphole of the plug P, while pressure is kept from downhole locations. When used during fracture operations, for example, the plug T may isolate pressures of 10,000 psi or so. Depending on the type of plug P used, an internal ball B may be contained in the plug P, or a separate ball may be deployed to seat on the plug P.
As will be appreciated, any slipping or loosening of the plug P can compromise operations. Therefore, it is important that the slips 12a-b sufficiently grip the inside of the casing. At the same time, however, the plug P and most of its components are preferably composed of millable materials because the plug P is milled out of the casing once operations are done, as noted previously. As many as fifty such plugs P can be used in one well and must be milled out at the end of operations. Therefore, having reliable plugs P composed of entirely of (or mostly of) millable material is of particular interest to operators.
Wicker slips (e.g., 12b) are made of metal, and composite slips (e.g., 12a) have inserts 13 typically made from cast or forged metal. For example, the inserts 13 may also be composed of carbide, which is a dense and heavy material, or even ceramic. When a plug P having composite slips (e.g., 12a) with carbide inserts 13 is milled out of the casing, the inserts 13 tend to collect in the casing and are hard to float back to the surface. In fact, in horizontal wells, the carbide inserts 13 may tend to collect at the heel of the horizontal section and cause potential problems for operations. Given that a well may have upwards of forty or fifty composite plugs P used during operations that are later milled out, a considerable number of carbide inserts 13 may be left in the casing and difficult to remove from downhole. Similar issues occur of course when the slips (e.g., 12b) are metallic and milled out due to the metal remnants left in the well.
Various types of plugs have been used for many years as evidenced, for example, by U.S. Pat. Nos. 5,398,763 and 9,033,041. In fact, the interest in plugs for wellbore tubulars has been (and will continue) to be of vital importance to operators. To that end, the subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above and to improving the types, uses, performance, and the like of plugs for wellbore tubulars.